1. Field of the Invention
The present invention relates to a belt-conveyor device and an image forming apparatus that includes the belt-conveyor device.
2. Description of the Related Art
A tandem system is widely employed in color image forming apparatuses such as color printers and color copiers. In the tandem system, a plurality of photoconductive drums are arranged in the travel direction of an endless transfer belt, a toner image is formed by adhering toners of different colors such as yellow, magenta, cyan, and black on an electrostatic latent image formed on each of the photoconductive drums, and the toner image of each color is transferred to the transfer belt in turn. A transfer belt driven by a belt-conveyor device sometimes gets biased or sometimes meanders, when travelling, in the direction orthogonal to its travel direction (in the width direction of the transfer belt). The meander of the transfer belt causes a relative position displacement of each toner image and a reduction in the image quality. Therefore, there is a need to have a means for controlling the transfer belt so that it does not meander.
There is known a system, as a method of controlling meander of a transfer belt, that controls an angle of inclination of a steering roller from a reference surface. The steering roller being one of that rollers that support the transfer belt (hereinafter, “steering system”). There is also known another system that adjusts and controls a tilt of an adjusted roller in a proportional relationship with respect to an amount of displacement in a position of an intermediate transfer belt (for example, see Japanese Patent Application Laid-open No. 2002-287527). In these systems, a transfer belt has a smaller load than in a system in which an edge of the transfer belt is guided to control a bias and is superior itself in durability. However, it is necessary to detect a belt position in the steering system, a transfer belt is likely to meander unless an angle of inclination of a steering roller is set based on a grasped correct belt position.
As described above, when printing starts while the transfer belt is meandering, relative position displacement occurs in toner images of yellow, magenta, cyan, and black, which causes a reduction in image quality. Furthermore, when meandering of the transfer belt does not converges for a long time, it takes long time to turn on an image forming apparatus or to become a printing possible condition from an operation of an image forming apparatus recovering from error, thereby causing a reduction in printing efficiency. Therefore, it is necessary to correct meander of a transfer belt and to return the meandering belt to an original condition as soon as possible.
With regard to detection of a belt position, because a shape of an edge of a transfer belt is nonlinear, the detected signal has a displacement component due to the shape of the edge. Thus, when detecting the belt position, the displacement component due to the shape of the edge of the transfer belt needs to be removed. The technologies that are currently available for removing the displacement component are as follows:
(1) A technology that averages belt position data by a cycle worth of a belt during driving a transfer belt to calculate a belt position (hereinafter, “a first conventional technology”) (for example, see Japanese Patent Application Laid-open No. H11-193143).
(2) A technology that calculates a current belt position by arranging a belt home and its detecting unit and comparing the previously extracted shape of an edge of a transfer belt with detected belt position data, serving the belt home as a reference, to detect a reference position in the belt-travel direction (hereinafter, “a second conventional technology”) (for example, see Japanese Patent Application Laid-open No. H10-139202). It needs time for a transfer belt to end a cycle at the maximum after starting driving the transfer belt to accurately get hold of a belt position in the first and the second conventional technologies.
Because time for a transfer belt to end a cycle is required, it is necessary to take time for a transfer belt to make a half cycle on average. Therefore, a transfer belt is likely to meander in a period to accurately get hold of a belt position. It needs time to converge meander velocity due to meander of a transfer belt. When the converging time of the meander velocity is long, it takes long time to turn on an image forming apparatus or to become a printing possible condition from an operation of an image forming apparatus recovering from error, thereby causing a reduction in printing efficiency.
Under a condition of not converging meander velocity, when stops of a transfer belt due to error occurrence are consecutively repeated many times and meander correction generated by activation of the transfer belt is not completed, another meander is generated, leading to an accumulation of the meander amount. As a result, durability of the transfer belt is considerably reduced, for example, the transfer belt comes into contact with a frame of a belt-conveyor device and is damaged.